Enteropeptidase is a serine protease that converts trypsinogen, which is secreted from the pancreas after a meal, into trypsin. Trypsin in a state activated by enteropeptidase then activates protease precursors such as chymotrypsinogen, procarboxypeptidase, and proelastase. These active forms of proteases degrade dietary proteins into amino acid units, and the resulting amino acids are absorbed from the small intestine. Thus, enteropeptidase inhibitors are capable of suppressing protein degradation and absorption and are useful as therapeutic drugs for obesity.
Examples of heterocyclic compounds include the following:
(1) A compound that has a trypsin inhibitory action and is useful in the treatment or prophylaxis of renal diseases and diseases involving trypsin, the compound being represented by the following formula:

wherein
ring A represents

R1 represents H, halogen, lower alkyl, or OH;
R2 represents H, optionally substituted cycloalkyl, optionally substituted aryl, an optionally substituted aromatic heterocyclic ring, an optionally substituted non-aromatic heterocyclic ring, —C(O)-lower alkylene-optionally substituted aryl, or optionally substituted lower alkyl;
L1 represents —Y1-lower alkylene-Y2— or —C(O)—N(R6)—;
Y1 represents a bond or —C(O)—;
Y2 represents a bond, —N(R6)—, or —C(O)—N(R6)—;
L2 represents -(lower alkylene optionally substituted by CO2H or the like)-, —Y3-cyclohexanediyl-Y4—, or —Y3-phenylene-Y4—, or L2 may form optionally substituted cyclic amino together with R2;
Y3 represents a bond or lower alkylene;
Y4 represents a bond, lower alkylene, or —C(O)—;
R3 represents H, lower alkyl optionally substituted by halogen, halogen, OH, —O-lower alkyl, cycloalkyl, aryl, or the like;
R4 represents lower alkyl optionally substituted by halogen, halogen, OH, —O-lower alkyl, cycloalkyl, aryl, or the like;
R5 and R6 each represent H or lower alkyl;
X1, X2, and X3 each represent CH or N, provided that at least one of X1, X2, and X3 is N;
m represents an integer of 0 to 4;
p represents an integer of 0 to 3; and
q represents an integer of 0 to 4
(Patent Literature 1).
(2) A compound that has a serine protease inhibitory action and is useful in the treatment or prophylaxis of obesity, hyperlipidemia, diabetes mellitus, diabetic complications, and metabolic syndrome, the compound being represented by the following formula:

wherein
R1, R2, R3, and R4 each represent H or the like;
HetAr represents an optionally substituted heteroaromatic ring;
X represents optionally substituted lower alkylene or the like;
Y represents carbonyl or the like;
A represents

or the like; and
R6 and R7 each represent H, optionally substituted lower alkyl, or the like
(Patent Literature 2).
(3) A compound that has a serine protease inhibitory action and is useful in the treatment or prophylaxis of obesity, hyperlipidemia, diabetes mellitus, diabetic complications, and metabolic syndrome, the compound being represented by the following formula:

wherein
D represents a benzene ring, a naphthalene ring, or a pyridine ring;
Het represents a heterocyclic ring;
R1 represents H or the like;
R2 represents nitro, lower alkyl, or the like;
X represents optionally substituted lower alkylene;
Z represents —N(R3)- (wherein R3 represents H, optionally substituted lower alkyl, optionally substituted lower cycloalkyl, or the like);
Y represents a single bond or —(CH2)p-C(R4a) (R4b)-(CH2)q- (wherein R4a and R4b each represent H, lower alkyl, or aralkyl, and p and q each represent an integer of 0 to 5); and
A represents —CO2R6 (wherein R6 represents H or lower alkyl) or

(wherein Q represents optionally substituted lower alkylene, and R7 represents H or lower alkyl)
(Patent Literature 3).
(4) A compound that has an enteropeptidase inhibitory action and is useful in the treatment or prophylaxis of obesity and diseases associated with abnormal fat metabolism, the compound being represented by the following formula:

wherein
B represents boron;
W represents a nitrogen-containing functional group

X represents a linker (CX1X2)p;
Y and Z each represent OH, OR (wherein R represents alkyl), a homocyclic ring, a heterocyclic ring, or the like;
R1 represents aminoacyl, acyl, or the like; and
R2 represents H, alkyl, or OR (wherein R represents H or alkyl)
(Patent Literature 4).
(5) A compound that has a serine protease inhibitory action and is useful in the treatment or prophylaxis of obesity, diabetes mellitus, etc., the compound being represented by the following formula:

wherein
R1 and R2 each represent alkyl or the like; and
X represents —OR3, —NR4R5, or the like
(Patent Literature 5).